The legality of the benefit cap as it applies to many carers of disabled people is under challenge at the High Court.

Test cases are being brought by two families where adults are providing full-time care for elderly and disabled grandmothers.

The families argue that the cap is "unfair and unlawful" because of its impact on the vulnerable and those who help them.

Their lawyers will contend before a judge in London that carers like them, who receive a carers' allowance, are able to perform their caring roles only with the support of state benefits to cover housing and living expenses.

They cannot work because, to qualify for the allowance, they have to provide full-time care - upwards of 35 hours a week - to a severely disabled person who receives disability living allowance (DLA).

The secretary of state for work and pensions, Iain Duncan Smith, is criticised for exempting actual DLA claimants from the cap - but not the carers themselves.

Only those who care for children or a spouse are exempt from the cap.

Solicitor for the families, Rebekah Carrier, said: "My clients have been hit by the benefit cap because they are disabled or they provide essential care to their disabled relatives. They are not skivers - they are strivers.

"They provide full time care and save the state money. The Government seeks to justify the cap by the financial savings achieved but the long term consequences of this arbitrary benefit cap are likely to have not only devastating consequences for individual disabled people and those who care for them, but serious financial costs.

"The Supreme Court has already ruled that the benefit cap breaches international protections for the rights of children.

"Now the High Court is considering whether it also breaches the rights of disabled people and their carers. The Government must halt this policy which simply hits vulnerable people."

Campaigners argue that "u nwaged" carers save the Government at least £119 billion per year.

They condemn the care allowance as "a pittance" at £62.10 a week for a minimum 35 hours and say it is hard to qualify for.

A Department for Work and Pensions spokesman said: "The department cannot comment on ongoing court proceedings.

"The benefit cap is a vital reform that ensures we have a welfare system that is fair for those who need it and those who pay for it.

"All households with someone in receipt of a disability-related benefit are exempt from the benefit cap, recognising the extra costs disability can bring."